Physicians use various prostheses to correct problems associated with the cardiovascular system, especially the heart. For example, the ability to replace or repair heart valves with prosthetic devices has provided surgeons with a method of treating heart valve deficiencies due to disease and congenital defects. A typical procedure involves removal of the native valve and surgical replacement with a prosthetic heart valve.
Prosthetic heart valve leaflets or occluders perform the function of opening and closing to regulate blood flow through the heart valve. Typically, heart valve leaflets must either pivot or flex with each cycle of the heart to open and close. Heart valves function as check valves, which open for flow in one direction and close in response to pressure differentials.
Prostheses can be constructed from natural materials such as tissue, synthetic materials or a combination thereof. Prostheses formed from purely synthetic materials can be manufactured, for example, from biocompatible metals, ceramics, carbon materials, such as graphite, polymers, such as polyester, and combinations thereof. Heart valve prostheses with purely synthetic materials can be manufactured with rigid occluders or leaflets that pivot to open and close the valve, or flexible leaflets that flex to open and close the valve.
Although mechanical heart valves with rigid pivoting occluders have the advantage of proven durability through decades of use, they are associated with blood clotting on or around the prosthetic valve and thromboembolism. Blood clotting can lead to acute or subacute closure of the valve. For this reason, patients with mechanical heart valves remain on anticoagulants for as long as the valve remains implanted. Anticoagulants have associated risks and cannot be taken safely by certain individuals.
Heart valve prostheses with flexible leaflets can be constructed with tissue leaflets or polymer leaflets. In prostheses with flexible leaflets, the leaflets function similarly to natural leaflets. While the leaflets are flexible, they must have a well defined and stable configuration to properly open and close the valve at each cycle in response to pressure differentials. Also, the leaflets should be durable to provide stable performance over many years of use.
Unlike mechanical valves, tissue based bioprostheses do not require the long term use of anticoagulants due to a lower incidence of thromboembolism. While tissue leaflets have desired flexibility and acceptable hemodynamic performance, tissue leaflets can calcify after implantation, which results in loss of flexibility, resulting in improper closure and/or opening of the valve. For individuals with appropriate indications, tissue-based heart valve prostheses provide practical alternatives to mechanical heart valve prostheses.
Valve prostheses with polymer leaflets have the potential to overcome the shortcomings of both tissue and mechanical valve designs. The polymers incorporated into heart valve prostheses should provide long term stable function to be suitable alternatives for tissue leaflets or mechanical valve leaflets.